Washington health officials are investigating several instances of a rare and fatal birth defect in Yakima County last year.

It occurs when a baby’s brain doesn’t develop as it should during the first month of pregnancy. The Washington state Department of Health expects one case among the roughly 4,000 births in the county each year. There were eight cases reported in 2012.

Health Department spokesman Donn Moyer said Wednesday that the investigation will take several months. He also says the reported cases are dispersed across the county and are not located in one area.

The defect is generally caused by a lack of folic acid in the mother’s diet or by certain medications or hereditary conditions. Women with diabetes, smokers, obese women and Hispanic women are also at higher risk.